The present invention relates to a liquid crystal display device, and more particularly to an active matrix type liquid crystal display device using thin film transistors (TFT).
In an active matrix type liquid crystal display device, a TFT (thin film transistor) is used as a semiconductor operating element (switching element). An inverted staggered structure is generally used for the structure of the TFT, wherein gate electrodes are formed on a substrate and signal lines and other electrodes are formed thereon. Practically, scan signal lines (such as gate lines and gate electrodes) are formed on a transparent insulating substrate, a gate insulating layer is formed thereon, a semiconductor layer is formed on the upper portion of the gate insulating layer, drain electrodes (data lines) and source electrodes are formed on the semiconductor layer, and transparent pixel electrodes are connected to the source electrodes. An image signal voltage is supplied to the drain electrodes (data lines). A liquid crystal display device using such a TFT structure is disclosed, for instance, in JP-A-2-48639 (1990).
When a driving circuit is formed on a periphery of the substrate, any of two kinds of mounting methods, such as TCP (Tape Carrier Package) and COG (Chip On Glass) methods, have been used.
Conventionally, the TCP method has been used more widely than the COG method. However, currently, the COG method is becoming to be used more widely in view of its superiority in cost reduction, finer pitch formation, and other advantages.
In accordance with the COG method, there are three kinds of connections, such as a connection of the input terminal of the driving circuit chip with the substrate, a connection of the output terminal of the driving circuit chip with the substrate, and a connection of the substrate with a FPC (Flexible Printed Cable) which is connected to an external circuit. And, an ACF (Anisotropic Conductive Film) is inserted at each of the above connections so that the connection is established via the ACF. In order to ensure reliability of the connection in the above case, an indium tin oxide (ITO) film, which is composed mainly of indium oxide to which tin oxide is added, is used as a wiring material of the substrate.
The liquid crystal display device using TFT technology has a feature that the display quality is superior, because active driving is possible. However, the process for forming a TFT onto the substrate is complex, and at least six separate photolithograpy processes are required. The reason for this is that six kinds of fundamental elements (film) are required for constructing the TFT, such as a scan signal line (gate lines), a gate insulating film, a semiconductor layer, a drain electrode (data line, generally as same as source electrode), a transparent pixel electrode, and a passivation layer, and patterning of each of the above films is performed by photolithography. Furthermore, because the thickness of all of the above films is several hundreds nm, and the width of the line formed by the patterning is small, such as in the order of 10 xcexcm, breakage of the lines (particularly, the data line and transparent pixel electrode) readily occurs, when they are contaminated with foreign particles, or at a portion crossing over a step on the base, and, if the patterning is not performed precisely in the photolithography process, a short circuit between electrodes or lines is readily created. If surface contamination during the process is added as a further consideration, it is seen that the electrical contact resistance between the source electrode and the transparent pixel electrode, and the gate line, or the data line, and the transparent pixel electrode at the terminal portion around the substrate, is increased significantly. Once the above phenomena are generated, defects in the display of data by the liquid crystal display device result. That means that the production yield of the substrate is decreased, and the production cost is increased.
As one of the conventional methods of increasing the production yield, a structure has been proposed wherein an insulating film between layers (passivation layer) is provided on the source electrode and the drain electrode of the TFT, and the source electrode and the drain electrode are connected with the pixel electrode via an opening provided in the insulating film between layers. In such a structure, a short circuit between the source electrode and the drain electrode with the pixel electrode, which is readily created when these electrodes are formed on the same plane, can be prevented.
However, in the conventional liquid crystal display device, decreasing the contact resistance at the connecting portion of each of the elements, which form conductive paths for electrical signals, has not been considered thoroughly when mounting the driving circuit, the TFT, and other elements on the substrate of the liquid crystal display device, and, particularly, a problem, such as low production yield of the terminal connecting portion, has not been solved. That is, in order to decrease the contact resistance when the driving circuit chip is connected with the drain line, the drain line is composed of Cr (chromium), an ITO film is arranged at the surface of the driving circuit chip, and the drain line and the driving circuit chip are contacted all through the surface so as to increase the contact area. On the other hand, the area of the region, other than the displaying region of the display panel (peripheral area), is required to be decreased. However, if the area of the contacting portion is decreased in order to meet this requirement, the contact resistance can not be decreased, because the specific contact resistance (the contact resistance per unit area) between ITO and Cr is high. If the area of the contacting portion is increased, water is liable to enter through a defect, such as a pin-hole and the like, in the resin used for packaging, and degradation of the contacting portion may readily occur.
One of the objects of the present invention is to provide an active matrix type liquid crystal display device, which is capable of decreasing the contact resistance at the contacting portions between each of the elements forming the conductive paths for signals.
In accordance with the present invention, the active matrix type liquid crystal display device comprises a liquid crystal layer including a liquid crystal material which forms a plurality of pixels, a pair of substrates, at least one of which is transparent, the substrates being arranged to face each other while holding the liquid crystal layer therebetween, means for generating scan pulses, means for generating image data, a plurality of scan lines, which are arranged dispersedly on one substrate of the pair of substrates and are connected to the means for generating scan pulses, and a plurality of data signal lines, which are arranged so as to intersect with the plurality of scan lines in a manner to form a matrix and are connected to the means for generating image data, and further, comprising, in a plurality of display regions surrounded by the plurality of the scan lines and the plurality of the date signal lines, transparent pixel electrodes arranged on one substrate of the pair of substrates, counter electrodes arranged to face the transparent pixel electrodes holding the liquid crystal layer therebetween and supplied with a liquid crystal driving voltage, and
an insulating film which covers the respective semiconductor active elements for driving the pixels connected to the scan lines, the data signal lines, and the transparent pixel electrodes, the respective scan lines, the respective data signal lines, and the respective semiconductor active elements for driving the pixels, wherein
at least the material of one element forming the connecting portion of the elements, which compose the signal transfer lines related to the scan lines and the data signal lines, is an alloy of at least a chemical element selected from the group consisting of Nb, Mo, Ta, and W, with Cr,
the scan lines and the means for generating scan pulses are connected to each other via a first opening formed in the insulating film, the data signal lines and the means for generating image data are connected to each other via a second opening formed in the insulating film, a polycrystalline thin film, which is connected to the means for generating scan pulses, is inserted into the first opening, and a polycrystalline thin film, which is connected to the means for generating image data, is inserted into the second opening, and the polycrystalline thin film is composed of indium tin oxide, which is made of mainly indium oxide and added tin oxide, having a specific resistance of, at the utmost, 6xc3x9710xe2x88x924 xcexa9cm.
Furthermore, in accordance with another feature of the present invention, the active matrix type liquid crystal display device comprises a liquid crystal layer including a liquid crystal which forms a plurality of pixels, a pair of substrates, at least one of which is transparent, arranged to face each other with the liquid crystal layer interposed therebetween, means for generating scan pulses, means for generating image data, a plurality of scan lines, which are arranged dispersedly on one substrate of the pair of substrates and connected to the means for generating scan pulses, and a plurality of data signal lines, which are arranged so as to intersect with the plurality of the scan lines in a manner to form a matrix and are connected to the means for generating image data, and
further comprising, in a plurality of display regions surrounded by the plurality of the scan lines and the plurality of the data signal lines, transparent pixel electrodes arranged on one substrate of the pair of substrates,
counter electrodes arranged to face the transparent pixel electrodes with the liquid crystal layer interposed therebetween and supplied with a liquid crystal driving voltage, and
an insulating film which covers respective semiconductor active elements for driving the pixels connected to the scan lines, the data signal lines, and the transparent pixel electrodes, respective scan lines, respective data signal lines, and respective semiconductor active elements for driving the pixels, wherein
at least the material of one element of the elements forming the connecting portion of the elements, which compose the signal transfer lines related to the scan lines and the data signal lines, is an alloy of at least a chemical element selected from a group consisting of Nb, Mo, Ta, and W, with chromium (Cr),
the scan lines and the means for generating scan pulses are connected vie a first opening formed in the insulating film, the data signal lines and the means for generating image data are connected via a second opening formed in the insulating film, electrodes of the semiconductor active element for driving a pixel and the transparent pixel electrodes are connected via a third opening formed in the insulating film, a polycrystalline thin film, which is connected to the means for generating scan pulses, is inserted into the first opening, a polycrystalline thin film, which is connected to the means for generating image data, is inserted into the second opening, and a polycrystalline thin film, which is connected to the transparent pixel electrodes, is inserted into the third opening, and the polycrystalline thin film is composed of indium tin oxide, which is made of mainly indium oxide and added tin oxide, having a specific resistance of, at the utmost, 6xc3x9710xe2x88x924 xcexa9cm.
In composing the above liquid crystal display device, the following features can be added:
(1) The content of the chemical element which forms the alloy film with chromium is in the range from 20 to 80% by weight.
(2) The scan lines and the means for generating scan pulses are formed via a plurality of the first openings formed along the scan lines, and the data signal lines and the means for generating image data are connected to each other via a plurality of the second openings formed along the data signal lines.
(3) The specific contact resistance of the polycrystalline film and the alloy at respective ones of the first opening, the second opening, and the third opening is, at the utmost, 1xc3x97105 xcexa9xcexcm2.
(4) Scan pulse transmitting lines, which are arranged in other regions than the display region on the substrate as lines belonging to the means for generating scan pulses and for transmitting the scan pulses, are connected with other elements by an anisotropic conductive film, image data transmitting lines, which are arranged in other regions than the display region on the substrate as lines belonging the means for generating image data and transmitting the image data, are connected with other elements by the anisotropic conductive film, polycrystalline thin film is formed on the plane of the scan pulse transmitting lines connecting the other elements and of the image data transmitting lines connecting the other elements, and the polycrystalline thin film is composed of indium tin oxide, which is made of mainly indium oxide and added tin oxide, having a specific resistance of, at the utmost, 6xc3x9710xe2x88x924 xcexa9cm.
In accordance with the means described above, the contact resistance at the connecting portion can be decreased, because at least one element of the elements forming the connecting portion of the elements, which compose the signal transfer lines, is made of an alloy of at least a chemical element selected from a group consisting of Nb, Mo, Ta, and W, with chromium (Cr). Therefore, the production yield of the circuit elements can be improved even if the area of the mounting region of the circuit element is decreased.